Most drivers have heard the term “running rich”. Admittedly, many of us don’t know what it means; we only know it’s not good.

When we start our car, a series of reactions take place. The efficiency of these reactions dictates how “rich” your car runs; if your vehicle runs rich, it means there’s an issue in your engine.

A car that runs rich causes a few problems: higher fuel costs, a poorer driving experience, and a worse environmental impact.

Below, we describe what running rich is, why it’s a problem and the long-term impacts of ignoring this issue.

Running rich vs lean: what it means

So, what does “running rich” mean, and how does it happen?

Running rich only occurs if there is an issue with your engine’s combustion process. The cause of this is your engine receiving too much fuel. Conversely, “running lean” refers to your car receiving too much air and not enough fuel.

With this in mind, is it better to have your car be running rich vs lean?

Both issues are as bad as each other. Ideally, there would be an even distribution of fuel-to-air, through what is referred to as an even “stoichiometric ratio”. This ratio is determined when your car is manufactured, and is optimised for a balance between performance and reliability.

Running rich symptoms

You don’t need to visit a mechanic to check if your car runs rich. There are a few clear rich fuel mixture effects that any driver can spot.

A strong, pungent smell

The strongest symptom is a strong, pungent smell from your engine. If you’re not sure what it smells like, then don’t worry, your car probably is not running rich – it’s a bit like rotten eggs.

Essentially, this smell comes from uncombusted fuel. As mentioned, running rich is the result of a faulty combustion process, and this leftover fuel escapes into the exhaust in all of its stinky, polluted glory.

Usually, our catalytic converters prevent the smell from appearing. However, very rich fuel is impossible for the converter to break down, leading to the emissions of unprocessed fuel.

We’ve covered catalytic converters in our in-depth blog: Catalytic converter theft: why it's on the rise and how to prevent it.

Higher fuel costs

Do you feel like you’re filling up your car more? Does it seem like it costs more to fill up the tank?

Well, higher fuel costs is a clear symptom of running rich. It implies that your car is not combusting fuel efficiently.

However, it’s important to note that other factors can lead to increased fuel consumption, including colder weather.

Unpredictable idling

How does your car feel when it’s idle? Is it smooth and comfortable? Or does it feel rough and cause vibration?

If idling in your car causes vibration, then running rich is likely the cause. The RPMs on the tachometer may be erratic, implying an issue is occurring with combustion.

Notice your engine light

Your engine light provides you with a lot of information. Each car model will have a diagnostic code that triggers when too much exhaust gas is produced in the combustion chamber of the engine.

This is the key issue that causes running rich. Other dashboard warnings from oxygen sensors, pressure sensors and mass air flow sensors can trigger warnings too.

So, pay attention to your dashboard notices – they are there for a reason.

Ruined spark plugs

Now, the majority of these symptoms occur when driving. However, if you pop open the bonnet, you can really see if your car runs rich.

For example, spark plugs full of black, dry soot means excessive carbon deposits are forming. This condition implies combustion is not occurring correctly, leading to too many carbonates in the system.

If left untreated, the spark plugs will eventually clog up the catalytic converter, which is very expensive to replace.

There are other symptoms that a trained mechanic can test. Higher carbon monoxide emissions, faulty oxygen and airflow sensors, and underperforming fuel pressure regulation are amongst the symptoms professionals notice.

How to fix a car running rich

Okay, so let’s say your car has a few of the above symptoms. Does that mean it’s running rich? Well, no – you need proof.

Proving it requires a professional with an OBD scanner. We’ve covered the importance of OBD ports in our blog: What is an OBD port and what are they used for?

At a mechanics, they will usually:

  1. Run full diagnostics to find the exact cause of the problem.
  2. Change the car’s air duct flap.
  3. Clean the mass airflow sensor.
  4. Vacuum hoses and lines.
  5. Change faulty spark plugs.
  6. Review the oxygen sensor.
  7. Replace the catalytic converter.
  8. Fix the engine.

Of course, it’s doubtful that your car will require all of the above. It depends on the damage, and if your engine is completely broken, then it may be better to scrap the car and purchase a new one.

More motoring tips, from MotoNovo

Now that you know the signs of running rich, you’ll be well equipped to react quickly if you spot them in the future.

Of course, running rich isn’t the only thing you need to be mindful of when choosing a car. It’s equally as important to stay clued up on the latest news and guidance, including the ins and outs of motor finance. To do just that, stay tuned to our motoring blog.